In the field of drug discovery there is a constant need for novel scaffolds that enable the rational design of potentially bioactive molecules. Carbohydrates have recently come under scrutiny as offering a source of scaffolds that allow for a high degree of substitution, and offer access to both functional and structural diversity. The nature of monosaccharide molecules is such that there are numerous different stereoisomers available that can provide access to a greater degree of molecular space than do the scaffolds presently employed in drug discovery.
Carbohydrate monomers predominantly contain hydroxyl groups but also may contain other functionalities such as an amino and/or carboxylate function. In essence, the concepts involved in drug discovery through carbohydrate based molecular and structural diversity, are twofold: (1) The primary concept involves the exploitation of the high functional density found around the carbohydrate ring to display several different moieties of biological relevance. There is a dual significance to this substitution in that (i) the substituents relative position around the ring may be varied in relation to each other and, (ii) each individual moiety may be substituted for a class of such moieties and therefore themselves may be varied (by example: an arginine mimetic may be substituted at position 1, 2, 3, 4 or 5 around a ring in relation to other peptidomimetics, by the same token the arginine mimetic may represent a class of different arginine bioisosteres which may all be similarly substituted). (2) The second concept involves exploiting the structural diversity inherent in carbohydrate isomers. Each of the substituents around a carbohydrate ring may theoretically be presented in either an axial or equatorial configuration allowing access to hugely diverse molecular space. Many monosaccharides are naturally occurring, which aside from being useful in their own right, present themselves as cheap starting materials to access more exotic configurations.
There are other factors that promote carbohydrates as useful building blocks for drug discovery, for example the relative positions of the functional groups on the sugar rings are conveniently spaced such that they can effectively enable mimicry of (for example), peptide motifs such as peptidic turns and loops, as well as cyclic peptides.
The major difficulty encountered in attempts to employ monosaccharides as scaffolds, is associated with monosaccharide chemistry. In the past carbohydrate chemistry was considered arduous, protracted and not cost effective. Particularly, the degree of orthogonal protection group chemistry required to allow free access to any one of a monosaccharide's functional groups (usually five) was deemed too high to ever be effected in a commercially viable manner. As a corollary, the more easily effected peptide synthesis only requires a maximum three orthogonal protecting groups, additionally the conditions required for peptide synthesis are often milder, thus peptide synthesis has so far been able to be effected more easily than carbohydrate synthesis. Fortunately, recent developments in synthetic carbohydrate chemistry have begun to allow regular access to carbohydrates as molecular scaffolds. In a recent patent application (PCT AU00100025) we disclosed a range of orthogonally protected building blocks suitable for oligosaccharide synthesis. The building blocks presented in this application are also suitable for use as intermediates in the synthesis of compounds of the present invention, and represent compounds and methods which define the state of the art.
A large number of Carbohydrate based templates and scaffolds has now been published in the scientific literature. A review of the major contributions by Gruner et. al., (Chem. Rev., 2002, 102, p 491-514) highlights this activity. Within the general literature, there are two distinct types of carbohydrate templates (i) sugar amino acids and (ii) carbohydrate scaffolds.
Sugar amino acids are carbohydrates which contain both an amine function and a carboxylic acid function, and are used in place of amino acids in peptide type syntheses. The synthesis of monosaccharides for this purpose is exemplified by the work of Fleet (Tetrahedron, 1996, 52, p10711; Tetrahedron Assym., 1996, 7, p387; Tetrahedron Assym., 1996, 7, p157) and Le Merrer (Tet. Lett., 1995, 36, p6887) for furanoid sugars, and by Dondoni (J. Org. Chem., 1994, 59, p6404), Vogel (J. Carbohyd. Chem., 1994, 13, p37) and Kessler (see chem rev. above) for pyranoid sugars.
Sugar amino acids have been used in peptide synthesis, and in the formation of linear oligomers for various biological purposes (see chem reviews above). Importantly, all of these compounds contain an amino function and a carboxylate function directly attached to the carbohydrate ring, and these functional groups are involved in amide bond forming processes which is the central concept in their use. The compounds of this type are distinctly different from the compounds of the present invention.
Carbohydrate scaffolds have also received considerable attention in the scientific literature, at least by way of desideratum. In concept, these compounds provide a chiral scaffold on which pharmaceutically active moieties are presented. This is the field of the present invention which adds to and is distinct from the state of the art.
The use of carbohydrates as scaffolds was promulgated by Hirschmann and co workers (Hirschmann et. al., J. Am. Chem. Soc., 114, 9217-9218, 1992) who employed this concept to develop a potent NK-1 receptor antagonist (Hirschmann et. al., J. Am. Chem. Soc., 115, 12550-12568, 1993), (Hirschmann et. al., J. Med. Chem., 39, 2441-2448, 1996). The fundamentals of this work have also been patented by Hirschmann et. al. (PCT/US1994/012233).
In a similar manner, Papageorgiou et al, have applied the concept to furanoid structures, developing weak somatostatin inhibitors in the process (Papageorgiou et. al., Bioorg. Med. Chem. Lett., 2, 135-140, 1992).
Weak inhibitors of integrin receptors and endothelin receptors have also been developed by applying this concept (Nicolaou, K. C., et. al, Tetrahedron, 1997, 53, p8751; Moitessier, N., et. al., Lett. Pep. Sci., 1998, 5, p75; Moitessier, N., et. al., Bioorg. Med. Chem., 2001, 9, p511.).
A number of other research groups have developed libraries of compounds based on this scaffold principle, and these groups are referred to in Gruner's review (vide supra). Despite the plethora of work to date, the compounds disclosed above have three common features which distinguish them from the current work: (i) all of the substituents are attached to the scaffold through an oxygen linkage, (ii) the anomeric position is always an O glycoside, and (iii) all of the available hydroxyl positions are substituted.
These features, when taken together, place significant limitations on the utility of the compounds. For example, ether linkages provide considerable rotational freedom and it is generally accepted that rotational freedom often results in diminished biological activity (Murphy et. al., J. Org. Chem., 68, 5692-5704, 2003). To this end, the present invention is directed to carbohydrate templates which have one or two amines directly attached to the carbohydrate ring, allowing the introduction of, for example, amide linked, sulfonamide linked, urea linked and carbamoyl linked moieties with significantly reduced rotational freedom and often better physical properties.
In a similar manner, the requisite for all of the positions to be substituted can lead to compounds of higher lipophillicity, higher molecular weight and lower solubility without imparting greater biological activity. In the present invention we disclose compounds with one or two hydroxyl positions unsubstituted, allowing generally improved solubility characteristics and lower molecular weights that would be expected for the corresponding fully substituted molecules.
These two features represent significant improvements over compounds described in the literature and are the result of considerable new method developments by the inventors.
Of all the carbohydrate scaffold work reported in the scientific and patent literature to date, we have found few examples of amine containing scaffolds outside the sugar amino acid class. Kunz et. al. (WO 99/07718) have claimed 2-deoxy 2-amino sugars as scaffolds for drug discovery. This citation does not teach or exemplify a compound with an amine group directly attached to the ring in the two position or any other position.
The disclosures in Kunz's relate specifically to the use of glucose, galactose and mannose as scaffolds and the methods described are not generally applicable to other monosaccharide scaffolds. In contrast, the compounds of the present invention are all O glycosides which are further limited by a narrow range of unsubstituted substituents dictated by the low reactivity of the sugar hydroxyls under the synthetic conditions disclosed. It is apparent that this technology displays significant disadvantages to the present invention; the efficiencies of conversion, the range of potential substituents, the various inversion chemistries that introduce both alternate oxy and amino stereochemical orientations, and the versatile alkylative chemistries of the present invention represent significant improvements over the methods of Kunz's application. Particularly, the present invention provides stereoisomers of monosaccharides that have a nitrogen or a carbon atom attached to the ring in positions 3,4,5 and 6 of a monosaccharide or tetrahydrofurano/pyrano ring system. Of particular interest to the medicinal chemist is the inclusion of linking functionalities that are likely to be stable to physiological conditions thus allowing the drug to reach the desired target intact, or in an active form.
Despite the general paucity of amine containing carbohydrate scaffolds in the literature, there are many examples of monosaccharide building blocks and protected aminosugars employed for oligosaccharide synthesis. By way of example, U.S. Pat. No. 4,818,816 discloses a compound 1-methyl-2-carbobenzyloxy,3-benzyl glucosamine, a monosaccharide building block used in the synthesis of synthetic heparinoid oligomers. The compounds of the present invention represent a significant departure from the simple building block type aminosugars, both in the diversity and complexity which is achievable. In order, to further distinguish the compounds of the present invention from the prior art, the use of standard amine protecting groups in carbohydrate synthesis is specifically excluded.
Sabesan (U.S. Pat. No. 5,220,008) discloses a series of higher oligosaccharides as inhibitors on influenza. Within the claims of this patent, a partially protected monosaccharide (structure IV) is also disclosed. The compounds of this structure are protected monosaccharides for oligosaccharide synthesis which are known in the art and do not represent compounds for drug discovery.
Similarly, Alchemia Pty Ltd has disclosed in PCT/AU01/01307 building blocks, methods of syntheses, and final products relating to the employment of monosaccharide compounds as drug like molecules. The compounds of PCT/AU01/01307 are specifically directed at inhibitors of the muramyl cascade of enzymes and are hereby excluded from specification by the incorporation of this reference. A number of other publications relating to muramyl type compounds have appeared in the literature. Liu et. al. (Biorg. Med Chem Lett., 10, 2000, 1361-1363) present a series of compounds containing a benzyl glycoside at the anomeric position, an acetate at C-2 and a peptide homologated lactate at C-3 of a glucosamine scaffold. These compounds and those disclosed by Xiao (Peptides: Biol and Chem., Proc. 5th Int. Chinese Peptide Symp., 1998 CA: 134:178795) represent compounds and methods which help define the art of carbohydrate chemistry but are not directly relevant to the present invention.
It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.